LEDs found to have multiple uses on multiple crops

Researchers at Purdue University are finding LEDs can
have positive effects on both ornamentals and leafy vegetables.

By David Kuack

As more research is done with light emitting diodes
(LEDs), scientists are discovering new ways to use the lights on ornamental and edible
plants. Researchers at Purdue University have done extensive studies on annual
bedding plants, comparing the growth of seedling plugs and vegetative cutting liners
under LEDs.
“My goal is to continue to do research with LEDs because
we are finding new and exciting results, especially with the indoor production
of young plants and microgreens,” said associate horticulture professor Roberto
Lopez. “Some of the work that we have been doing has shown the benefits of
“If you would have asked me two years ago if I would ever
try to produce plugs indoors and not in a greenhouse, I would have said no. If
you would have asked me five years ago if I would be working on greens or
vegetables, I would have said no. Now I am doing both of those things with

Purdue University graduate student Joshua Craven
and associate horticulture professor Roberto Lopez are
studying the effects LED
lights have on ornamental
plants and leafy vegetables.
Photo by Tom Campbell, Purdue University

No need for

Lopez and former graduate student Wesley Randall found
that greenhouse-grown seedling plugs of impatiens, marigold, petunia, vinca and
zonal geranium did as well or better when supplemented with LEDs compared to
plugs supplemented with light from high pressure sodium lamps. What Lopez found
surprising was the quality of the plugs produced in a growth room with LEDs as
the only light source.

“LEDs produce better plugs when they’re grown indoors
than when they are grown in a greenhouse with sunlight supplemented with light
from LEDs or high pressure sodium lamps,” Lopez said. “It is amazing how good
the plugs look grown in an indoor multilayer production system with LEDs. The
plugs are compact, sturdier and greener with a similar root and shoot dry mass
to greenhouse-grown plants supplemented with light from LEDs or high pressure
sodium lamps.”
One crop that Lopez said they are still “tweaking” with
LEDs is petunias.
“Petunias, which are long day plants, when moved from an
indoor grow room equipped with red and blue LEDs, encountered a slight delay in
flowering in the greenhouse,” he said. “We are going to see if exposing the
plants to far-red LED light prior to moving them into the greenhouse will
induce them to flower.”
Using LEDs to
intensify leaf, flower color

Lopez said many of the annual spring bedding plants grown
in greenhouses in northern climates are produced under low light levels. The
result is that some plants don’t produce the same intense foliage colors that
they would if they were grown outdoors.

“Plants grown in glass greenhouses are not exposed to the
sun’s ultraviolet light because it is blocked by the glass,” he said. “The
result is that crops like zonal geraniums and purple fountain grass (Pennisetum setaceum ‘Rubrum’) don’t
“color up” like they would outdoors. One of the things we noticed with zonal
geraniums was the dark patterns on the leaves stood out much more when the
amount of blue light was increased. We hypothesized and found it was a result
of an increase in anthocyanin production. We have also looked at geraniums that
have very dark foliage and found not only does leaf color darken, but flower
color can be made darker by exposing market-ready plants to red:blue LEDs.”
Lopez said the change in leaf color due to anthocyanin
production was also dramatic for purple fountain grass.
“Purple fountain grass is a very popular ornamental
species produced by many growers,” Lopez said. “Grown in the greenhouse, the
leaves appear to be dull green and not very purple. We found that putting the
plants under a combination of red and blue LEDs for one to two weeks of what we
are calling “end-of-production lighting” resulted in an attractive purple
color. UV light is what stimulates anthocyanin synthesis.”
He said in the case of purple fountain grass, only the
leaves exposed to the LED lights change color. Those leaves not exposed to the
LED light remain green.
Expanding studies
to vegetable crops

Seeing the positive results that occurred with LEDs and
purple fountain grass, Lopez and PhD student W. Garrett Owen expanded the
research to red leaf lettuce to see if they could produce a similar response.

“Trying to produce red leaf lettuce can be difficult for
greenhouse growers if they are producing crops under low daily light integrals
(DLIs),” Lopez said. “Growers producing red leaf lettuce under low DLIs are
essentially producing green lettuce.
“We placed red leaf lettuce under the same LED treatments
used for purple fountain grass and the plants colored up in three to five days.
Based on our research, red leaf lettuce and purple fountain grass can be placed
under a 50-50 red and blue LED combination prior to harvesting or shipping
triggering anthocyanin formation.”
on Purdue University research,
red leaf lettuce can be placed under a
50-50 red
and blue LED combination prior
to harvesting triggering anthocyanin formation
the lettuce‘s red color.
Photo courtesy of Roberto Lopez,
Purdue University
Based on the results related to LEDs and anthocyanin
formation, Lopez said the studies may be expanded to look at the impact of LED
light on ornamental cabbage and kale. “Growers, especially those in the South,
have a hard time coloring up ornamental cabbage and kale,” he said. “It is
primarily a temperature response, as the night temperatures get cooler the
plants start to color up.”
Lopez and Owen did a small study placing ornamental cabbage
and kale under LEDs that resulted in a minimal color change. When
greenhouse-grown plants were grown under cool night temperatures and exposed to
LEDs, they exhibited the most intense color.
“What we are proposing is for growers in warmer climates
who have access to coolers, is to use a cool temperature/LED treatment,” he
said. “We will be conducting this study next fall. Smaller container sizes like
4-inch pots, could be rolled on carts into a cooler and exposed to cool
temperatures and LED lights for three to four days prior to shipping enabling
the plants to color up.”

Another study conducted by graduate students Joshua
Gerovac and Joshua Craver looked at the effect of LEDs on the growth of three
different microgreen species (kohlrabi, mustard and mizuna) in an indoor
multilayer production system. The study included three different light
qualities and three different DLIs (light quantity).

“Overall what we have seen is as the DLI increases, this
is for three microgreen species we trialed, the length of the hypocotyl,
basically the height of the microgreen, decreases,” Lopez said. “The more light
the plants are provided, the more compact they are. If the plants received 6
moles of light, they were much taller than if they received 18 moles of light.
Depending on the growers’ market, some customers might want microgreens that
are a little leggier or they might want plants that are more compact. That will
depend on market preference.”
The ideal LEDs

Lopez said the ideal vertical LED light module would
contain all of the wavelength colors.

“The vertical LED light with all the different colors
would enable growers to turn them on when they need them and off when they
don’t, depending on the stage of plant growth,” he said. “Once flowering begins
a grower doesn’t want stem elongation. Far-red light works for flowering so the
far-red would be turned on for the minimum amount of time required for
flowering. If the grower wants to increase the amount of anthocyanin in the
leaves or flowers, he can turn on the red and blue light near the end of the
crop. To be able to turn on specific colors when a growers needs them, that is
something I envision happening with LEDs.”

For more:
Roberto Lopez, Purdue University, Department of Horticulture and Landscape
Architecture; (765) 496-3425; rglopez@purdue.edu;

David Kuack is a freelance technical writer in Fort
Worth, Texas; dkuack@gmail.com.

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